Process for improving the devolatilization of polymer slurry produced in a polymerization reactor

ABSTRACT

The present invention relates to a process for improving the devolatilization of polymer slurry produced in an olefin polymerization reactor. The process is characterized in that it involves an adjustment of the temperature of the interior surface of the flash line for periodically transferring polymer slurry from the polymerization reactor to a downstream processing unit. In particular, in accordance with the present process, the temperature of the interior surface of the flash line is adjusted to a temperature which is equal to or higher than the softening temperature of the polymer passing through said flash line whereby said temperature is higher in a first half of the length of the flash line than in the remaining length of the flash line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2007/061547, filed Oct.26,2007, which claims priority from EP 06123169.2, filed Oct 30, 2006.

TECHNICAL FIELD

The present invention relates to the field of olefin polymerization. Inparticular, the present invention relates to a process for improving thedevolatilization of polymer slurry produced in an olefin polymerizationloop reactor.

BACKGROUND

Polyethylene (PE) is synthesized by polymerizing ethylene (CH₂═CH₂)monomers. Because it is cheap, safe, stable to most environments andeasy to be processed polyethylene polymers are useful in manyapplications. According to the properties polyethylene can be classifiedinto several types, such as but not limited to LDPE (Low DensityPolyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (HighDensity Polyethylene). Each type of polyethylene has differentproperties and characteristics.

Ethylene polymerizations are frequently carried out in a loop reactorusing monomer, liquid diluent and catalyst, one or more optionalco-monomer(s), and hydrogen. The polymerization in a loop reactor isusually performed under slurry conditions, with the produced polymerusually in a form of solid particles which are suspended in the diluent.The slurry in the reactor is circulated continuously with a pump tomaintain efficient suspension of the polymer solid particles in theliquid diluent. Polymer slurry is discharged from the loop reactor bymeans of settling legs, which operate on a batch principle to recoverthe slurry. Settling in the legs is used to increase the solidsconcentration of the slurry finally recovered as product slurry. Theproduct slurry is further discharged through heated flash lines to aflash tank, where most of the diluent and unreacted monomers are flashedoff and recycled. The polymer particles are dried, additives can beadded and finally the polymer may be extruded and pelletized.

In passing from the reactor to the flash tank the pressure drops and thepolymer slurry is partially devolatilized, i.e. the volatile componentsthereof are removed. Insufficient devolatilization of polymer slurry mayultimately result in polymer end products that contain an undesiredamount of diluent and/or unreacted reactants, such as e.g.(co-)monomer(s). Devolatilization is improved with higher temperature ofthe polymer slurry resulting in lower content of volatiles in thepolymer. However, high temperatures in the flash lines induce foulingproblems, i.e. sticking of polymer product to the interior walls of theflash lines, and degradation of the polymer particles in the flashlines. On the other hand, in the flash lines where temperatures are toolow, devolatilization of the polymer slurry turns out to beinsufficient, resulting in polymer end products having unsatisfactoryproperties and qualities.

U.S. Pat. No. 3,428,619 discloses a method for transferring polymerslurry from a polymerization reactor to a flash tank via a transfer linewherein the liquid diluent is substantially vaporized and the polymerparticles are dried during the transfer. The transfer line comprises aplurality of externally heated zones, for instance two or four zones, ofvarying size. The method comprises controlling the flow rate of thematerials flowing through the different zones. The external heating ofthe zones is carried out at a temperature sufficiently elevated tovaporize the liquid present on the solid particles but which is belowthe softening temperature of the polymer particles.

In view of the above, there remains a need in the art for providing animproved process for the devolatilization of polymer slurry issued froma polymerization reactor.

The present invention aims to provide an improved process for thedevolatilization of polymer slurry produced in a polymerization reactor.More in particular, the present invention aims to provide a processwherein devolatilization of polymer is improved.

SUMMARY

In a first aspect, the present invention therefore relates to a processfor improving the devolatilization of polymer slurry produced in apolymerization reactor. The process comprises the steps of:

introducing ethylene monomer, one or more optional olefin co-monomer(s),and a diluent into a loop reactor,

feeding at least one polymerization catalyst into the reactor,

polymerizing said ethylene monomer and said optional co-monomer(s) toproduce a polymer slurry comprising essentially liquid diluent and solidethylene polymer particles,

periodically withdrawing said polymer slurry from said polymerizationreactor, and

passing said withdrawn polymer slurry through a heated flash line into aflash tank, said flash line having an interior surface.

The process is in particular characterized in that the temperature ofthe interior surface of the flash line is adjusted to a temperaturewhich is equal to or higher than the softening temperature of thepolymer passing through said flash line, whereby said temperature ishigher in a first half of the length of the flash line than in theremaining length of the flash line.

The term “periodically” withdrawing or discharging polymer as usedherein refers to a process wherein there is a periodical withdrawal ordischarge of polymer slurry from the polymerization reactor. Inaccordance with the present invention, the polymer slurry is dischargedthrough one or more settling legs, and transferred through one or moreheated flash lines into a flash tank.

The softening temperature of the polymer passing through said flash lineis measured according to standard ISO 306:2004, method A50. The meltingpoint of the polymer passing through said flash line is measuredaccording to standard ISO 3146:2004, method A50.

The present invention provides a proper design of the temperature offlash lines in order to improve polymer slurry devolatilization. Inprior art systems, polymer slurry is heated in flash lines to atemperature which is below its softening temperature in order to avoidabove-mentioned fouling and degradation problems. In contrast, inaccordance with the present invention polymer slurry is heated in aflash line to a temperature which is equal to or higher than itssoftening temperature without inducing fouling or degradation problems.By adjusting the temperature of the interior surface of flash linesimproved vaporization is obtained of unreacted reactants present in thepolymer slurry, such as diluent and/or (co)-monomer(s). The presentprocess thus provides improved separation of unreacted reactants fromthe polymer particles.

Furthermore, in accordance with the present process a mixture of gas andpolymer solids is obtained that leaves the flash lines to enter a flashtank. The polymer solids leave the flash lines at a higher temperature.Such polymer solids are in consequence easier to further devolatilize indownstream processes. The need for expensive downstream flashing anddrying operations of the obtained polymer is therefore significantlyreduced, since substantial vaporization already occurs in the flash linezone and also because of the higher temperature of the polymer particlesleaving the flash line.

In a second aspect, the invention relates to a flash line forperiodically transferring a polymer slurry from a polymerization reactorto a flash tank comprising a pipe which is adapted to receive saidpolymer slurry, and which is provided with a jacket surrounding saidpipe, said jacket being adapted for receiving a heating fluid, wherebysaid flash line comprises control means for adjusting the temperature ofthe interior surface of the pipe to a temperature which is equal to orhigher than the softening temperature of the polymer passing throughsaid pipe, and which is higher in a first half of the length of theflash line than in the remaining length of the flash line.

In a preferred embodiment the flash line according to the invention isconnected with one or more settling legs, and preferably with at leasttwo settling legs, for periodically transferring polymer slurry fromsaid settling legs to a flash tank.

The present invention will now be disclosed in further detail hereunder.The description is only given by way of example and does not limit theinvention. The reference numbers relate to the hereto-annexed FIGURE.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a process and apparatuses forthe devolatilization of polymer slurry produced in a polymerizationreactor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is especially applicable to the polymerizationprocess of ethylene. Suitable “ethylene polymerization” includes but isnot limited to homo-polymerization of ethylene or co-polymerization ofethylene and at least one olefin co-monomer in presence of apolymerization catalyst. Ethylene polymerization comprises feeding to areactor the reactants including the monomer ethylene, one or moreoptional co-monomer(s), a diluent, a catalyst, optionally a co-catalyst,and a terminating agent such as hydrogen.

Olefin co-monomers which are suitable for being used in accordance withthe present invention may comprise but are not limited to aliphaticC₃-C₂₀ alpha-olefins.

Examples of suitable aliphatic C₃-C₂₀ alpha-olefins include propylene,1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene,1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. In a preferredembodiment of the present invention, said co-monomer is 1-hexene.However, it should be clear from the present invention that otherco-monomers may as well be applied according to the present invention.

Diluents which are suitable for being used in accordance with thepresent invention may comprise but are not limited to hydrocarbondiluents such as aliphatic, cycloaliphatic and aromatic hydrocarbonsolvents, or halogenated versions of such solvents. The preferredsolvents are C₁₂ or lower, straight chain or branched chain, saturatedhydrocarbons, C₅ to C₉ saturated alicyclic or aromatic hydrocarbons orC₂ to C₆ halogenated hydrocarbons. Nonlimiting illustrative examples ofsolvents are butane, isobutane, pentane, hexane, heptane, cyclopentane,cyclohexane, cycloheptane, methyl cyclopentane, methyl cyclohexane,isooctane, benzene, toluene, xylene, chloroform, chlorobenzenes,tetrachloroethylene, dichloroethane and trichloroethane. In a preferredembodiment of the present invention, said diluent is isobutane. However,it should be clear from the present invention that other diluents may aswell be applied according to the present invention.

As used herein, the term “polymerization slurry” or “polymer slurry” or“slurry” means a substantially two-phase composition including liquidand polymer solids. The solids include catalyst and a polymerizedolefin, such as polyethylene. The liquids may comprise an inert diluent,such as isobutane, with dissolved monomer such as ethylene, a molecularweight control agent such as hydrogen, optional co-monomer(s), one ormore antistatic agents, antifouling agents, scavengers, or other processadditives.

Suitable catalysts and co-catalysts for use in the polymerization ofethylene are well known in the art. The polymerization catalyst used incombination with the present invention can for example be ametallocene-based catalyst, a Ziegler-Natta catalyst or a chromiumcatalyst.

The present invention relates to a process for improving thedevolatilization of polymer slurry produced in a polymerization loopreactor. The process involves an adjustment of the temperature of theinterior surface of the flash line. The term “flash line” as used hereinrefers to an elongated jacketed pipe, the interior of which is heatedindirectly by running a heating medium through the jacket. The polymerslurry is vaporized in the pipe utilizing the heat supplied fromcondensing fluid in the jacket surrounding the pipe. The fluid may besteam, which flows through the jacket in a direction which iscountercurrent to the flow direction of the polymer slurry in the pipe.In a preferred embodiment, the flash line comprises control means foradjusting the temperature of the interior surface of the pipe to atemperature which is equal to or higher than the softening temperatureof the polymer passing through said pipe.

The flash line may be divided in different flash line zones,corresponding to different pipe parts, having a same or a differentdiameter. Each of said flash line zones may comprise control means foradjusting the temperature of the interior surface of the pipe of saiddifferent flash line zones. In particular, each pipe part may beprovided with a jacket and heated using the heat supplied from the fluidin the jacket. Preferably the flash line comprises at least 3, andpreferably at least 4 zones. In a preferred embodiment the flash linecomprises between 4 and 10 flash line zones, and for instance 5, 6, 7, 8or 9 zones.

The temperature of the interior surface of the flash line or a zonethereof corresponds to the temperature of the interior surface of thepipe of the flash line or a zone thereof. The temperature of theinterior surface of the flash line or a zone thereof will be adjusted toa temperature which is equal to or higher than the softening temperatureof the polymer passing through said flash line. The control means foradjusting the temperature of the interior surface of the pipe of theflash line (zones) comprise means for adjusting temperature, pressureand/or flow of the heating fluid heating said flash line (zones).

While polymer slurry flows through the flash line, it will be heated andvaporization occurs at least partially within the flash line. Inaccordance with the invention, a large portion of the liquid of thepolymer slurry is vaporized by the time the polymer material reaches theflash tank. The material flowing through the flash line near thepolymerization reaction will thus mainly comprise liquid and polymersolids, while the material flowing through the flash line near the flashtank will mainly comprise vapour (gas) and polymer solids.

Surprisingly, despite the fact that the polymer slurry in the flash lineis heated to a temperature which is above its softening point, thepolymer does not accumulate and/or agglomerate in the flash lines. Noplugging problems of the flash lines occur. A mixture of vapor andheated polymer solid particles arrives at the flash tank, in which thevapors are separated and removed from the heated dry polymer particles.The particles are collected for further drying and processing asdesired.

In one embodiment, the temperature of the interior surface of the flashline may be the same in different zones of the flash line.

In a preferred embodiment, the temperature of the interior surface ofthe flash line varies in different zones thereof. For instance, thetemperature of the interior surface of the flash line may be higher inzones located near the reactor outlet than in zones located near theflash tank inlet. In a particularly preferred embodiment, the inventionrelates to a process wherein the temperature of the interior surface ofthe flash line is higher in the first half, and preferably the firstthird of the length of the flash line than in the remaining length ofthe flash line. With the terms a or the “first third” or a or the “firsthalf” of the length of the flash line is meant that part of the flashline that is closest to the polymerization reactor. Preferably, thetemperature of the interior surface of the flash line in the first half,and preferably in the first third of the length of the flash line is upto 70%, 60%, 50%, 40%, 35%, 30%, 25%, 20%, 15%, or 10% higher than thetemperature in the remaining length of the flash line.

In a preferred embodiment, a process is provided according to theinvention, wherein the temperature of the interior surface of the flashline is up to 10% higher, and for instance up to 10%, 15%, 20%, 25%,30%, 35%, 40%, 50%, 60% or 70% in the first half, and preferably thefirst third, of the length of the flash line than in the remaininglength of the flash line.

In a preferred embodiment, the process according to the presentinvention, comprises periodically withdrawing (or discharging) saidpolymer slurry from said polymerization reactor by means of one or moresettling legs, and passing said withdrawn (or discharged) polymer slurryfrom said settling legs through one or more heated flash lines into aflash tank. Preferably the invention provides a process comprisingperiodically withdrawing polymer slurry from a polymerization reactor bymeans of at least two settling legs, and for instance by means of 2, 3,4, 5, 6, 7, 8, 9, or 10 settling legs, and passing said withdrawnpolymer slurry from said settling legs through one or more heated flashline, and for instance to through one single heated flash line, into aflash tank.

The temperature of the interior surface of the flash line or a zonethereof is adjusted by adjusting pressure, temperature, and/or flow ofthe fluid heating said flash line or a zone thereof.

In one embodiment, the process comprises adjusting the temperature ofthe interior surface of the flash line by adjusting the pressure of theliquid, preferably steam, heating said flash line. Preferably, the steampressure is at most 2.5 barg, and for instance 0.5, 0.7, 0.9, 1.0, 1.1,1.5, 2 or 2.5 barg. The term “barg” as used herein stands for bar gaugeand gives the difference between atmospheric pressure and an internalpressure (bar=barg+1). In one preferred embodiment, the present processcomprises keeping the pressure of the liquid, preferably steam, heatingsaid flash line at a maximum pressure of for instance 2.5 barg along thelength of the flash line. In another embodiment, however, the presentinvention may also relate to a process wherein the pressure of the fluidheating the flash line or a zone thereof is higher in the first half,and preferably the first third of the length of the flash line than inthe remaining length of the flash line. For instance, the pressure ofthe fluid heating the flash line in the first half, and preferably inthe first third of the length of the flash line is up to 70%, 60%, 50%,40%, 35%, 30%, 25%, 20%, 15%, or 10% higher than the pressure of thefluid heating the flash line in the remaining length of the flash line.

In another embodiment, the process comprises adjusting the temperatureof the interior surface of the flash line by adjusting the temperatureof the liquid, preferably steam, heating said flash line. In a preferredembodiment, the steam temperature is up to 95, 100, 105, 110, 115, 120,125, 130, 135, or 139° C. The temperature of the fluid heating the flashline or a zone thereof is higher in the first half, and preferably thefirst third of the length of the flash line than in the remaining lengthof the flash line. For instance, the temperature of the fluid heatingthe flash line in the first half, and preferably in the first third ofthe length of the flash line is up to 70%, 60%, 50%, 40%, 35%, 30%, 25%,20%, 15%, or 10% higher than the temperature of the fluid heating theflash line in the remaining length of the flash line. In a preferredembodiment, the difference in temperature of the liquid (steam) heatingthe flash line and the softening temperature of the polymer transportedin the flash line is equal to or more than 10° C., and for instance morethan 11, 12, 13, 14 or 15° C.

In yet another embodiment, the process comprises adjusting thetemperature of the interior surface of the flash line by adjusting theflow of the liquid, preferably steam, heating said flash line. In apreferred embodiment, the steam flow is higher in the first half, andpreferably the first third of the length of the flash line than in theremaining length of the flash line. For instance, the flow of the fluidheating the flash line in the first half, and preferably in the firstthird of the length of the flash line is up to 70%, 60%, 50%, 40%, 35%,30%, 25%, 20%, 15%, or 10% higher than the flow of the fluid heating theflash line in the remaining length of the flash line.

In another preferred embodiment, the invention relates to a process asdefined herein, wherein the flash tank is operated at a pressure ofbetween 0.1 and 4 bar, and preferably at a pressure of between 0.2 and 3bar.

The present process may be applied in a double loop polymerisationreactor consisting of two liquid full loop reactors. The reactors areconnected in series by one or more settling legs of the first reactorconnected for discharge of slurry from the first reactor to said secondreactor. Polymer slurry issued from the second loop may be dischargedthrough flash lines to a product recovery zone, including a flash tank.The temperature of the interior surface of the flash lines can beadjusted in accordance with the invention to be at least the equal to orhigher than the softening temperature of the polymer particles in theline.

Referring now to FIG. 1 a polymerization loop reactor 1 is representedconsisting of a plurality of interconnected pipes. The vertical sectionsof the pipe segments 2 are provided with jackets 3. Polymerization heatcan be extracted by means of cooling water circulating in these jacketsof the reactor. Reactants are introduced into the reactor 1 by feedingline 4. Catalyst, optionally in conjunction with a co-catalyst oractivation agent, is injected in the reactor 1 by means of the conduct5. Normally the catalyst is introduced as a suspension in hydrocarbondiluent. The polymerization slurry is directionally circulatedthroughout the loop reactor 1 as illustrated by the arrows 6 by one ormore pumps, such as axial flow pump 7. The pump may be powered by anelectric motor 8. As used herein the term “pump” includes any devicefrom compressing driving, raising the pressure of a fluid, by means forexample of a piston or set of rotating impellers 9. The illustratedreactor 1 is further provided with two settling legs 10 connected to thepipes of the reactor 1. As the polymerization progresses polymer slurryaccumulates in these settling legs 10. The settling legs 10 are providedwith an isolation valve 11. These valves 11 are open under normalconditions and can be closed for example to isolate a settling leg fromoperation. Further the settling legs are provided with product take offor discharge valves 12. The discharge valves 12 may be any type ofvalve, which can permit continuous or periodical discharge of polymerslurry, when they are fully open. Polymer slurry settled in the settlinglegs 10 is removed by means of flash lines 13 to a product recoveryzone, including a flash tank 14.

As shown on FIG. 1, two settling legs are provided for allowing periodicdischarge of polymer slurry from the polymerization reactor and bothsettling legs are connected to a single flash line 13. The withdrawnpolymer slurry is passed from said two settling legs through a singleheated flash line into a flash tank. It shall however be understood thatmore than two and for instance 3, 4, 5, 6, 7, 8, 9, 10 settling legs canbe provided, that are all connected to one or more flash lines, and forexample to a single flash line.

Partial volatilization of unreacted reactants occurs in the flash lines.Unreacted reactants are further separated from the incoming polymersolids in the flash tank 14. Within the flash tank 14 unreactedreactants are removed as vapor via conduit 18. The gas flowing throughconduit 18 can be transferred to a recycle section 19, where thereactants in the gas are separated and if desired re-fed to thepolymerization reactor 1. Polyethylene powder is removed from the tank14 through conduit means 16 which conduct the polyethylene powder to apurge column 17. Herein, the PE powder is further treated to remove anyremaining (co-)monomer(s) and diluent. Treated PE powder is then removedthrough conduit means 20.

FIG. 1 represents a single flash line, although it shall be clear thatmore than one flash line may be provided to connect the reactor 1 with aflash tank 14. The flash line 13 is a conduit which comprises a seriesof jackets 15, 115, 215. The jackets delimit different flash line zones.In FIG. 1 three flash line zones are represented. However, more thanthree flash line zones may be provided on a flash line in accordancewith the present invention.

Advantageously, the present process and the present flash line can beoperated even when the flash tank is operated at a pressure of between0.1 and 4 bar, and preferably of between 0.2 and 3 bar, and for instanceof 0.4 bar, although the operational conditions of the flash lines areless flexible in view of such relatively low pressure conditions. Thepresent invention overcomes this difficulty and advantageously providesa process which enables to carefully adjust the operational conditionsof the flash line(s) and in particular the temperature of the interiorsurface of the flash line(s).

For example, during transfer from reactor 1 to flask tank 14 thepressure drops from e.g. about 40 bar to e.g. about 400 mbar, andpolymer slurry is partially devolatilized in the flash line 13. Furtherdevolatilization is effected by adjusting the temperature in the flashline 13. The temperature in the flash line is adjusted by adjusting thetemperature, pressure and/or flow, and preferably by adjusting thepressure of a heated fluid, e.g. steam, flowing through jackets 15, 115,215. The steam flows in a direction countercurrent to the direction thepolymer slurry flow in the flash line. The steam is introduced in thejackets 15, 115, 215 by lines 21, 121, 221, respectively, and withdrawnfrom the jackets by lines 22, 122, 222. The steam provides indirectheating of the polymer slurry in flash line 13, such that the polymerslurry is vaporized in the line 13. In accordance with the presentinvention, the temperature, pressure and/or flow, of the steam iscontrolled and adjusted such that the temperature of the interiorsurface 23 of the flash line 13 corresponds to or is higher than thesoftening temperature of the polymer particles in the line 13. When theflash line's interior surface is heated to such temperature, liquidvaporization in addition to vaporization effected by pressure reductionin line 13 is sufficient to volatilize a substantial part of the liquidpresent in the line 13. Preferably, the conduit 13 is of sufficientlength and diameter to permit transporting polymer slurry that expandsdue to pressure reductions and vaporization.

EXAMPLE

The following example illustrates that degassing of a PE polymer slurryproduced in a polymerization loop reactor can be improved by adjustingthe temperature of the interior surface of the flash line to atemperature which is equal to or higher than the softening temperatureof the polymer passing through said flash line.

In this example, a first PE slurry was passed through a flash linewherein temperature of the interior surface of the flash line was notadjusted. A second PE slurry was passed through a flash line whereintemperature of the interior surface of the flash line was adjusted byadjusting pressure of the fluid heating said flash line. In both cases,a mixture of gas and polymer solids was obtained that left the flashlines to enter a flash tank. Results of this comparative experiment aresummarized in Table 1.

TABLE 1 first PE slurry second PE slurry isobutane (kg/hour) 7195 5925PE (kg/hour) 9800 8990 Temperature in polymerization 90 90 reactor (°C.) Pressure in polymerization 39 39 reactor (barg) pressure in flashtank (barg) 0.41 0.40 action standard pressure +1 bar (g) profile flashtemperature (° C.) 58 51

It can be derived from Table 1 that the polymer solids that are derivedfrom the second PE slurry leave the flash lines at a higher temperaturethan the polymer solids derived from the first PE slurry. (Flashtemperature of 58° C. versus 51° C.—a gain of 7° C.). As a consequencethereof, it is easier and more efficient to further devolatilize polymersolids that are derived from the second PE slurry in downstreamprocesses.

1. A process for improving the devolatilization of polymer slurryproduced in a polymerization loop reactor comprising: introducingethylene monomer, one or more optional olefin co-monomers and a diluentinto a loop polymerization reactor; introducing at least onepolymerization catalyst into the reactor; polymerizing the ethylenemonomer and the optional co-monomer to produce a polymer slurrycomprising liquid diluent and solid ethylene polymer particles;periodically withdrawing the polymer slurry from the polymerizationreactor; and passing withdrawn polymer slurry through a heated flashline into a flash tank, wherein a temperature of an interior surface ofthe flash line is adjusted to a temperature which is equal to or higherthan a softening temperature of the polymer particles passingtherethrough and wherein the temperature of the interior surface ishigher in a first half of the length of the flash line than in aremaining length of the flash line, wherein the flash line comprises apipe that is provided with a jacket, and wherein the process comprisesflowing polymer slurry through the pipe and flowing a heating fluid forheating the flash line through the jacket, whereby the heating fluidflows in a direction which is countercurrent to the polymer slurry,wherein the loop polymerization reactor comprises a second loop reactorof a double loop reactor system.
 2. The process of claim 1, wherein theflash line comprises different flash line zones, and wherein thetemperature of the interior surface of the flash line varies in thedifferent zones.
 3. The process of claim 1, wherein the temperature ofthe interior surface of the flash line or a zone thereof is adjusted byadjusting temperature, pressure and/or flow of the fluid heating theflash line or a zone thereof.
 4. The process of claim 1, wherein thetemperature of the interior surface of the flash line is up to 10%higher in the first half of the length of the flash line than in theremaining length of the flash line.
 5. The process of claim 1 furthercomprising periodically withdrawing the polymer slurry from thepolymerization reactor through one or more settling legs; and passingthe withdrawn polymer slurry from the settling legs through one or moreheated flash lines into a flash tank.
 6. The process of claim 5, whereinthe one or more settling legs comprise at least two settling legs. 7.The process of claim 1, wherein the flash tank is operated at a pressureof between 0.1 and 4 bar.